A huge amount of gravity coming from a cluster of distant galaxies causes space to curl so much that light from them is curved and emanates from many directions. This “gravitational lens” effect allowed astronomers at the University of Copenhagen to observe the same star explode at three different locations in the sky. They predict that a fourth image of the same explosion will appear in the sky by 2037. The study, which has just been published in the journal Nature astronomy, offers a unique opportunity to explore not only the supernova itself, but the expansion of our universe.

One of the most fascinating aspects of Einstein’s famous theory of relativity is that gravity is no longer described as a force, but as a “curvature” of space itself. The curvature of space caused by heavy objects not only causes the planets to rotate around the stars, but can also bend the orbit of light beams.

The heaviest of all structures in the universe – galaxy clusters made up of hundreds or thousands of galaxies – can deflect light so far from distant galaxies behind them that they appear to be in a completely different place than they are. are actually.

But that’s not it: light can take several paths around a cluster of galaxies, which allows us to be lucky and make two or more observations of the same galaxy in different places in the sky. using a powerful telescope.

Supernova deja vu

Some routes around a cluster of galaxies are longer than others and therefore take longer. The slower the road, the stronger the gravity; yet another astonishing consequence of relativity. This scales the time it takes for light to reach us, and therefore the different images we see.

This wonderful effect enabled a team of astronomers from the Cosmic Dawn Center – a basic research center run by the Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark – as well as their partners international organizations, to observe a single galaxy in no less than four different locations in the sky.

Observations were made using the infrared wavelength range of the Hubble Space Telescope.

Analyzing data from Hubble, the researchers noted three bright light sources in a background galaxy that were evident in a previous round of observations from 2016, which disappeared when Hubble revisited the area in 2019. These three sources turned out to be multiple images of a single star whose life ended in a colossal explosion known as a supernova.

“A single star exploded 10 billion years ago, long before our own sun formed. The flash of light from this explosion has just reached us,” says associate professor Gabriel Brammer of the Cosmic Dawn Center, who led the study with Professor Steven Rodney. from the University of South Carolina.

The supernova, nicknamed “SN-Requiem”, can be seen in three of the galaxy’s four “mirror images”. Each image presents a different view of the development of the explosive supernova. In the last two images, it has yet to explode. But, by examining how galaxies are distributed within the galaxy cluster and how these images are distorted by curved space, it is in fact possible to calculate how “lagged” these images are.

This allowed astronomers to make a remarkable prediction:

“The fourth image of the galaxy is about 21 years late, which should allow us to see the supernova explode again, around 2037”, explains Gabriel Brammer.

Can teach us more about the universe

If we were to witness the explosion of SN-Requiem again in 2037, it would not only confirm our understanding of gravity, but also help shed light on another cosmological conundrum that has emerged in recent years, namely the expansion of our universe.

We know that the universe is expanding and that different methods allow us to measure how fast. The problem is that the different measurement methods do not all give the same result, even taking into account the measurement uncertainties. Could our observation techniques be flawed or, more interestingly, will we need to revise our understanding of fundamental physics and cosmology?

“Understanding the structure of the universe will be a top priority for major terrestrial observatories and international space organizations over the next decade. Studies planned for the future will cover much of the sky and are expected to reveal dozens, if not hundreds. rare gravitational lenses with supernovae like SN Requiem, ”says Brammer:

“Accurate measurements of the delays from such sources provide unique and reliable determinations of cosmic expansion and may even help reveal the properties of dark matter and dark energy.”

Dark matter and dark energy are the mysterious matter that is said to make up 95% of our universe, while we can only see 5%. The prospects for gravitational lenses are promising!


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